Very light axions are a generic prediction of string compactifications. If cosmic strings associated with these axions were produced in the early universe, they quickly approach a so-called scaling solution, such that strings persist in the sky today. I will present some remarkable signals of such strings coupled to photons. In this string background, there is a model-independent polarization rotation of CMB photons equal to alpha_em up to a rational number. This manifests itself as a rotation of E-modes in the CMB polarization to B-modes. The current CMB experimental sensitivity to this rotation is about 1%, with many orders of magnitude improvement expected for future experiments. I will show how measuring the undetermined rational number may shed light on the quantization of electric charge in the standard model. As the strings pass through galactic magnetic fields, they build up localized charge and current density traveling along the string. This sets up a cosmological plasma collider where collisions can occur at very high energies and produce interesting astrophysical signals.